How lengths are measured

Where a screw length is measured from depends on whether the head sits flush or stands proud. It is not the same point on every screw.

  • 1Flush heads — measured overall. A flat head, or a flat head with nibs, countersinks into the work. The measurement runs from the top of the head to the point.
  • 2Proud heads — measured under the head. A pan, truss, round washer, fillister or washer head sits on the surface. The measurement runs from the bearing surface under the head to the point.

Lengths are nominal

Screw lengths are nominal throughout the trade, and ours are no exception. A screw sold as 1 1/4 inch is manufactured to that nominal length within the tolerance of its class - it is a size designation, not a certified measurement. Do not use a nominal length to calculate a fit that has no margin in it.

Flush heads are measured overall

A flat head and a flat head with nibs are countersunk: the head disappears into the work, so the whole screw is inside the material. Those are measured overall - from the very top of the head to the point.

Proud heads are measured under the head

A pan, truss, round washer, fillister or washer head sits on top of the surface. Only the shank goes into the material, so the length that matters is measured from the bearing surface under the head to the point - not from the top of the head.

This is why two screws both labelled 1 inch can put different amounts of steel into the second piece. If you are working to a depth, check the head type first.

Rule of thumb: aim for roughly half the screw length in the second piece, and never let the point break the far face.

Every product page states which way that particular screw is measured, in the specification table.